Showing papers in "Oecologia in 1990"

The maturity index: an ecological measure of environmental disturbance based on nematode species composition

Abstract: Nematode assemblages constitute a potential instrument for assessing the quality of submersed, temporarily submersed, and terrestrial soils and for the development of an ecological typology and biomonitoring system. Interpretation of physical or pollution-induced disturbances has hitherto mainly been based on changes in diversity, dominance patterns or percentage of dorylaimids (Adenophorea). The maturity index, based on the nematode fauna, is proposed as a gauge of the condition of the soil ecosystem. Values on a coloniser/persister scale are given for nematodes that occur in The Netherlands. The possibilities of the use of this index are demonstrated by a retrospective interpretation of some literature data. The use of nematodes in environmental studies is discussed.

Leaf area ratio and net assimilation rate of 24 wild species differing in relative growth rate.

Abstract: Which factors cause fast-growing plant species to achieve a higher relative growth rate than slow-growing ones? To answer this question 24 wild species were grown from seed in a growth chamber under conditions of optimal nutrient supply and a growth analysis was carried out. Mean relative growth rate, corrected for possible ontogenetic drift, ranged from 113 to 356 mg g−1 day−1. Net assimilation rate, the increase in plant dry weight per unit leaf area and unit time, varied two-fold between species but no correlation with relative growth rate was found. The correlation between leaf area ratio, the ratio between total leaf area and total plant weight, and relative growth rate was very high. This positive correlation was mainly due to the specific leaf area, the ratio between leaf area and leaf weight, and to a lesser extent caused by the leaf weight ratio, the fraction of plant biomass allocated to the leaves. Differences in relative growth rate under conditions of optimum nutrient supply were correlated with the soil fertility in the natural habitat of these species. It is postulated that natural selection in a nutrient-rich environment has favoured species with a high specific leaf area and a high leaf weight ratio, and consequently a high leaf area ratio, whereas selection in nutrient-poor habitats has led to species with an inherently low specific leaf area and a higher fraction of root mass, and thus a low leaf area ratio.

The checkerboard score and species distributions.

Abstract: There has been an ongoing controversy over how to decide whether the distribution of species is “random” — i.e., whether it is not greatly different from what it would be if species did not interact. We recently showed (Roberts and Stone (1990)) that in the case of the Vanuatu (formerly New Hebrides) avifauna, the number of islands shared by species pairs was incompatible with a “random” null hypothesis. However, it was difficult to determine the causes or direction of the community's exceptionality. In this paper, the latter problem is examined further. We use Diamond's (1975) notion of checkerboard distributions (originally developed as an indicator of competition) and construct a C-score statistic which quantifies “checkerboardedness”. This statistic is based on the way two species might colonise a pair of islands; whenever each species colonises a different island this adds 1 to the C-score. Following Connor and Simberloff (1979) we generate a “control group” of random colonisation patterns (matrices), and use the C-score to determine their checkerboard characteristics. As an alternative mode of enquiry, we make slight alterations to the observed data, repeating this process many times so as to obtain another “control group”. In both cases, when we compare the observed data for the Vanuatu avifauna and the Antillean bat communities with that given by their respective “control group”, we find that these communities have significantly large checkerboard distributions, making implausible the hypothesis that their species distributions are a product of random colonisation.

Species effects on nitrogen cycling: a test with perennial grasses

Abstract: To test for differing effects of plant species on nitrogen dynamics, we planted monocultures of five perennial grasses (Agropyron repens, Agrostis scabra, Poa pratensis, Schizachyrium scoparium, and Andropogon gerardi) on a series of soils ranging from sand to black soil. In situ net N mineralization was measured in the monocultures for three years. By the third year, initially identical soils under different species had diverged up to 10-fold in annual net mineralization. This divergence corresponded to differences in the tissue N concentrations, belowground lignin concentrations, and belowground biomasses of the species. These results demonstrate the potential for strong feedbacks between the species composition of vegetation and N cycling. If individual plant species can affect N mineralization and N availability, then competition for N may lead to positive or negative feedbacks between the processes controlling species composition and ecosystem processes such as N and C cycling. These feedbacks create the potential for alternative stable states for the vegetation-soil system given the same initial abiotic conditions.

Countergradient variation in growth rate: compensation for length of the growing season among Atlantic silversides from different latitudes.

Abstract: How do organisms adapt to the differences in temperature and length of the growing season that occur with latitude? Among Atlantic silversides (Menidia menidia) along the east coast of North America, the length of the first growing season declines by a factor of about 2.5 with increasing latitude. Yet body size at the end of the first growing season does not decline. High-latitude fish must, therefore, grow faster within the growing season than do low-latitude fish. This geographical pattern has a genetic basis. Laboratory experiments on fish from six different locations revealed a latitudinal gradient in the capacity for growth (i.e., maximum growth potential). In two subsequent experiments using fish from Nova Scotia (NS), New York (NY) and South Carolina (SC) that had been separately reared in a common environment for several generations, differences in growth rate among populations were highly significant. The rank order was NS>NY>SC, but the difference among populations depended on temperature. High-latitude fish outperformed those from low latitudes primarily at the high temperatures that low-latitude fish would be expected to experience most often in nature. These results suggest that instead of being adapted for growth at low temperatures, fish from high latitudes are adapted for rapid elevation of growth rate during the brief interval of the year when high temperatures occur. Selection on growth rate results from sizedependent winter mortality: the importance to winter survival of being large increases with latitude but the length of the growing season simultaneously decreases. The end result is countergradient variation in growth rate, a phenomenon that may be much more widespread than currently recognized.

Causes and consequences of sea urchin abundance and diversity in Kenyan coral reef lagoons.

Abstract: Large differences in community structure of sea urchins and finfish have been observed in Kenyan reef lagoons. Differences have been attributed to removal of finfish predators through human fishing activities. This study attempted to determine (i) the major sea urchin finfish predators, (ii) the effect of predation on sea-urchin community structure, and (iii) the possible effect of sea urchin increases and finfish decreases on the lagoonal substrate. Six reefs, two protected and four unprotected, were compared for differences in finfish abundance, sea urchin abundance and diversity and substrate cover, diversity and complexity. Comparisons between protected and unprotected reefs indicated that finfish populations were ca. 4 x denser in protected than unprotected reefs. Sea urchin populations were >100 x denser and predation rates on a sea urchin, Echinometra mathaei, were 4 x lower in unprotected than in protected reefs. The balistidae (triggerfish) was the single sea-urchin finfish predator family which had a higher population density in protected than in unprotected reefs. Balistid density was positively correlated with predation rates on tethered E. mathaei (r=0.88; p<0.025) and negatively correlated with total sea-urchin density (r=-0.89; p<0.025) on the six reefs. We conclude from observations that the balistids Balistaphus undulatus and Rhinecanthus aculeatus are the dominant sea-urchin predators. The sea-urchin assemblage had its greatest diversity and species richness at intermediate predation rates and low to intermediate sea-urchin densities. At low predation rates and high sea-urchin density E. mathaei dominated the assemblage's species composition. Preferential predation on the competitive dominant maintains the assemblage's diversity, supporting the compensatory mortality hypothesis (Connell 1978) of coral reef diversity. Protected reefs had greater cover of hard coral, calcareous and coralline algae, and greater substrate diversity and topographic complexity than unprotected reefs which had greater algal turf and sponge cover. Coral cover and topographic complexity were negatively correlated with total sea urchin density. Although experimentation is lacking, these substrate changes may be due to the switch from finfish to sea-urchins as consumers which results from overfishing of finfish. Removal of top invertebrate-eating carnivores appears to have cascading effects on the entire coral reef ecosystem.

Remote sensing of the xanthophyll cycle and chlorophyll fluorescence in sunflower leaves and canopies

Abstract: Sudden illumination of sunflower (Helianthus annuus L. cv. CGL 208) leaves and canopies led to excess absorbed PFD and induced apparent reflectance changes in the green, red and near-infrared detectable with a remote spectroradiometer. The green shift, centered near 531 nm, was caused by reflectance changes associated with the de-epoxidation of violaxanthin to zeaxanthin via antheraxanthin and with the chloroplast thylakoid pH gradient. The red (685 nm) and near-infrared (738 nm) signals were due to quenching of chlorophyll fluorescence. Remote sensing of shifts in these spectral regions provides non-destructive information on in situ photosynthetic performance and could lead to improved techniques for remote sensing of canopy photosynthesis.

Soil water exploitation after fire: competition between Bromus tectorum (cheatgrass) and two native species.

Abstract: Causes for the widespread abundance of the alien grass Bromus tectorum (cheatgrass) after fire in semiarid areas of western North America may include: (1) utilization of resources freed by the removal of fireintolerant plants; and (2) successful competition between B. tectorum and individual plants that survive fire. On a site in northwestern Nevada (USA), measurements of soil water content, plant water potential, aboveground biomass production, water use efficiency, and B. tectorum tiller density were used to determine if B. tectorum competes with either of two native species (Stipa comata and Chrysothamnus viscidiflorus) or simply uses unclaimed resources. Soil water content around native species occurring with B. tectorum was significantly lower (P<0.05) than around individuals without B. tectorum nearby. Native species had significantly more negative plant water potential when they occurred with B. tectorum. Aboveground biomass was significantly higher for native species without B. tectorum. However, the carbon isotope ratio of leaves for native species with B. tectorum was not significantly different from individuals without B. tectorum. Thus, B. tectorum competes with native species for soil water and negatively affects their wate status and productivity, but the competition for water does not affect water use efficiency of the native species. These adverse effects of B. tectorum competition on the productivity and water status of native species are also evident at 12 years after a fire. This competitive ability of B. tectorum greatly enhances its capability to exploit soil resources after fire and to enhance its status in the community.

Nutrient use efficiency in evergreen and deciduous species from heathlands

Abstract: The nutrient (N, P) use efficiency (NUE: g g−1 nutrient), measured for the entire plant, of field populations of the evergreen shrubs Erica tetralix (in a wet heathland) and Calluna vulgaris (in a dry heathland) and the deciduous grass Molinia caerulea (both in a wet and a dry heathland) was compared. Erica and Calluna are crowded out by Molinia when nutrient availability increases. NUE was measured as the product of the mean residence time of a unit of nutrient in the population (MRT: yr) and nutrient productivity (A: annual productivity per unit of nutrient in the population, g g−1 nutrient yr−1. It was hypothesized that 1) in low-nutrient habitats selection is on features leading to a high MRT, whereas in high-nutrient habitats selection is on features leading to a high A; and that 2) due to evolutionary trade-offs plants cannot combine genotypically determined features which maximize both components of NUE.

Variation in foliar δ13C in Hawaiian Metrosideros polymorpha: a case of internal resistance?

Abstract: Sun leaves of Metrosideros polymorpha were collected in 51 sites on 9 lava flows that represented gradients of elevation, precipitation, substrate age, and substrate texture on Mauna Loa volcano, Hawai'i. Leaf mass per unit leaf area increased with increasing elevation on all flows, while foliar nitrogen concentration decreased with increasing elevation and increased with increasing substrate age. Foliar δ13C became less negative with increasing elevation on the wet east-side lava flows, but not the dry northwest-side flows; it did not reflect patterns of precipitation or presumed water availability. δ13C was very strongly correlated with leaf mass per area across all of the sites. Limited gas-exchange information suggested that calculated ci/ca did not decrease with elevation in association with less-negative δ13C, and photosynthesis per unit of nitrogen was significantly reduced in high-elevation plants. These results are consistent with a substantial internal resistance to CO2 diffusion in the thick Metrosideros polymorpha leaves in high elevation sites.

Sexual Selection Versus Alternative Causes of Sexual Dimorphism in Teiid Lizards

Abstract: The presence and extent of sexual dimorphisms in body form (size and shape) of adult macroteiid lizards were investigated. Males were significantly larger than females in the temperate species, Cnemidophorus tigris, and in the tropical species, Ameiva ameiva and C. ocellifer. Young adult C. tigris males grew faster than young adult females within and between reproductive seasons. Adult males of all species had larger heads than adult females of the same body size; this difference increased with body size. Moreover, male C. tigris were heavier than females of the same snout-vent length. The causes and consequences of the sexual dimorphisms were also examined. The possible causes of body size are especially numerous, and distinguishing the relative influences of the various causal selection factors on body size is problematical. Nevertheless, observational field data were used to tentatively conclude that intrasexual selection was the cause of larger body size of C. tigris males relative to females because (1) larger males won in male aggressive interactions, (2) the winning males gained access to more females by repelling competitors and by female acceptance, (3) larger males consequently had higher reproductive success, and (4) other hypothetical causes of larger male size were unsupported.

Biogeographic variation of food habits and body size of the America puma.

Abstract: The puma (Felis concolor) has the most extensive range of any terrestrial mammal in the Western Hemisphere, covering over 100° latitude. Food habits of different puma subspecies vary with latitude. Subspecies from temperate habitats generally eat larger prey and specialize on a smaller number of prey taxa, whereas, in tropical habitats, they prey on smaller, more varied prey. In North America, ungulates (primarily deer) represented 68% of the puma's diet by frequency of occurrence. Mean weight of vertebrate prey (MWVP) was positively correlated (r=0.875) with puma body weight and inversely correlated (r=-0.836) with food niche breadth in all America. In general, MWVP was lower in areas closer to the Equator. Patterns of puma prey selection are probably influenced by prey availability and vulnerability, habitat characteristics, and potential competition from the jaguar (Panthera onca).

Gorilla diet in the Lope Reserve, Gabon: A nutritional analysis

Abstract: The results of an analysis of gorilla diet in the Lope Reserve, Gabon are presented. Samples were assayed for nutrients and plant secondary compounds (total phenols, condensed tannins and alkaloids) in an attempt to explain gorilla food choice. The diet is the most diverse so far analysed for gorillas; it seems to be a balance between sugary fruit, proteinaceous leaves, and relatively fibrous stems. Most fruits and herbaceous stems are succulent, but some drier, fibrous fruit and bark is also consumed. Seeds are another component of the diet, including unripe ones. Fruit, seeds, leaves and bark may all contain very high levels of total phenols and condensed tannins; but all herbaceous stems assayed contain low levels of these compounds. Alkaloids are not apparently a significant component of gorilla foods, and may be avoided. Gorillas at Lope tend to avoid fatty fruit, and select leaves which are high in protein and low in fibre compared to the general vegetation. When fruit and preferred young leaves are scarce, proteinaceous barks and mature leaves, and sugary pith, are important sources of nutrients. We conclude that gorillas exploit the broad frugivore niche in West African lowland forests, and are part of the frugivore community there. What distinguishes them is their ability to eat large fibrous fruit, mature leaves and stems, and to overcome high levels of phenolics (we use “phenolics” as an umbrella term for both total phenols and condensed tannins). Gorilla diet at Lope overlaps greatly with that of sympatric, frugivorous, primates, and resembles more closely that of chimpanzees than it does gorilla diet studied elsewhere in Africa.

The role of litter in an old-field community: impact of litter quantity in different seasons on plant species richness and abundance.

Abstract: We studied the effect of removing and adding plant litter in different seasons on biomass, density, and species richness in a Solidago dominated old-field community in New Jersey, USA. We removed all the naturally accumulated plant litter in November (658 g/m2) and in May (856 g/m2) and doubled the amount of litter in November and May in replicated plots (1 m2). An equal number of plots were left as controls. Litter removal and addition had little impact on total plant biomass or individual species biomass in the growing season following the manipulations. Litter removal, however, significantly increased plant densities but this varied depending upon the season of litter removal, species, and life history type. Specifically, the fall litter removal had a much greater impact than the spring litter removal suggesting that litter has its greatest impact after plant senescence in the fall and prior to major periods of early plant growth in spring. Annual species showed the greatest response, especially early in the growing season. Both spring and fall litter removal significantly increased species richness throughout the study. Litter additions in both spring and fall reduced both plant densities and species richness in June, but these differences disappeared near the end of the growing season in September. We concluded than in productive communities where litter accumulation may be substantial, litter may promote low species richness and plant density. This explanation does not invoke resource competition for the decline in species richness. Finally, we hypothesize that there may be broad thresholds of litter accumulation in different community types that may act to either increase or decrease plant yield and diversity.

Comparative nutrient extraction from forages by grazing bovids and equids: a test of the nutritional model of equid/bovid competition and coexistence.

Abstract: Ruminants are unevenly distributed across the range of body sizes observed in herbivorous mammals; among extant East African species they predominate, in numbers and species richness, in the medium body sizes (10–600 kg). The small and the large species are all hind-gut fermenters. Some medium-sized hind-gut fermenters, equid perissodactyls, coexist with the grazing ruminants, principally bovid artiodactyls, in grassland ecosystems. These patterns have been explained by two complementary models based on differences between the digestive physiology of ruminants and hind-gut fermenters. The Demment and Van Soest (1985) model accounts for the absence of ruminants among the small and large species, while the Bell/Janis/Foose model accounts both for the predominance of ruminants, and their co-existence with equids among the medium-sized species (Bell 1971; Janis 1976; Foose 1982). The latter model assumes that the rumen is competitively superior to the hind-gut system on medium quality forages, and that hind-gut fermenters persist because of their ability to eat more, and thus to extract more nutrients per day from high fibre, low quality forages. Data presented here demonstrate that compared to similarly sized grazing ruminants (bovids), hind-gut fermenters (equids) have higher rates of food intake which more than compensate for their lesser ability to digest plant material. As a consequence equids extract more nutrients per day than bovids not only from low quality foods, but from the whole range of forages eaten by animals of this size. Neither of the current nutritional models, nor refinements of them satisfactorily explain the preponderance of the bovids among medium-sized ungulates; alternative hypotheses are presented.

The adaptive significance of an environmentally-cued developmental switch in an anuran tadpole.

Abstract: This study investigated the proximate basis of bimodally-distributed, environmentally-induced variation that occurs in natural populations of spade-foot toad tadpoles (Scaphiopus multiplicatus). Most individuals in most populations occur as a small, slowly-developing omnivore morph. In some of these same populations, a varying number of individuals occur as a large, rapidly-developing carnivore morph (Pfennig 1989). Censuses of 37 different natural ponds revealed that the frequency of the faster-developing carnivore morph correlated significantly positively with fairy shrimp density (their chief prey) and pond drying rate. By simultaneously varying two diet components and pond drying regime in artificial pools I found that only fairy shrimp density significantly affected the proportion of carnivores. Separate experiments established that the extent to which tadpoles developed the carnivore morphology correlated with shrimp density, and that morph determination depended on the ingestion of shrimp, not simply their presence. If a critical number of shrimp were ingested, the tadpole developed into a carnivore; if not, the tadpole developed by default into an omnivore. Thus a single cue — shrimp ingestion — triggers alternative ontogenetic trajectories. Using shrimp density to induce morph differentiation enables tadpoles to respond to their environment adaptively as shrimp are most abundant in highly ephemeral ponds, where the faster developing carnivores are favored.

Amphipods on seaweeds: partners or pests?

Abstract: Herbivorous marine amphipods have been implicated as important grazers on filamentous and ephemeral algae, and thus as beneficial to macrophytes in reducing overgrowth by epiphytic competitors. In North Carolina, USA, amphipods comprise 97% of all macroscopic animals inhabiting the abundant brown seaweed Sargassum filipendula, and peak in abundance between late winter and early summer. I used outdoor tank experiments to test the species-specific impact of common phytal amphipods on the growth of Sargassum and its epiphytes. The results show that seaweed-associated amphipods are a trophically diverse group that could either increase or decrease host fitness depending on their feeding preferences. The amphipods Ampithoe marcuzii, Caprella penantis, and Jassa falcata each significantly reduced growth of epiphytes on Sargassum plants relative to amphipod-free controls, while Ericthonius brasiliensis had no significant effect on Sargassum or its epiphytes. However, amphipod grazing was not necessarily beneficial to Sargassum. A. marcuzii consumed Sargassum in one outdoor tank experiment, reducing its mass by 11%, while Sargassum plants without amphipods grew by 81%. Epiphytes (mostly diatoms and the filamentous brown alga Ectocarpus siliculosus) and detritus remained abundant on these plants suggesting that A. marcuzii preferred the host to its epiphytes. Similarly, when given simultaneous access to Sargassum and to several common foliose and filamentous epiphytes in the lab, A. marcuzii ate Sargassum almost exclusively. The other three amphipods ate no macroalgae. In contrast to A. marcuzii, C. penantis consistently reduced epiphytes with no negative effect on Sargassum. Thus the species composition of the amphipod fauna can determine whether these animals increase or decrease seaweed fitness.

Factors influencing carbon fixation and water use by Mediterranean sclerophyll shrubs during summer drought

Abstract: Mediterranean sclerophyll shrubs respond to seasonal drought by adjusting the amount of leaf area exposed and by reducing gas exchange via stomatal closure mechanisms. The degree to which each of these modifications can influence plant carbon and water balances under typical mediterranean-type climate conditions is examined. Leaf area changes are assessed in the context of a canopy structure and light microclimate model. Shifts in physiological response are examined with a mechanistically-based model of C3 leaf gas exchange that simulates progressive reduction of maximum photosynthesis and transpiration rates and increasingly strong midday stomatal closure over the course of drought. The results demonstrate that midday stomatal closure may effectively contribute to drought avoidance, increase water use efficiency, and strongly alter physiological efficiency in the conversion of intercepted light energy to photoproducts. Physiological adjustments lead to larger reductions in water use than occur when comparing leaf area index 3.5 to 1.5, extremes found for natural stands of sclerophyll shrubs in the California chaparral. Reductions in leaf area have the strongest effect on resource capture and use during non-water-stressed periods and the least effect under extreme drought conditions, while shifts in physiological response lead to large savings of water and efficient water use under extreme stress. An important model parameter termed GFAC (proportionality factor expressing the relation of conductance [g] to net photosynthesis rate) is utilized, which changes in response to the integrated water stress experimence of shrubs and alters the degree to which stomata may open for a given rate of carbon fixation. We attempt to interpret this parameter in terms of physiological mechanisms known to modify control of leaf gas exchange during drought. The analysis helps visualize means by which canopy gas exchange behavior may be coupled to physiological changes occurring in the root environment during soil drying.

Consequences of herbivory in the mountain birch (Betula pubescens ssp tortuosa): importance of the functional organization of the tree.

Abstract: Three types of experiments indicate that the functional organization of the mountain birch may influence the ways in which the tree responds to simulated or natural herbivory. The first experiment showed that herbivory to both short and long shoot leaves affects plant development but, because growth largely proceeds by resources of the previous year, is manifested only in the year following the damage. The second experiment showed that even partial damage to a single long shoot leaf caused the axillary bud of that leaf to produce a shorter shoot the next year. Therefore, the value of a leaf depends also on the organ which it is subtending. In the third experiment we manipulated the apical dominance of shoots in ramets and caused improvement to leaf quality in extant shoots. Ramets within a tree responded individually, probably mediated by disturbance of the hormonal control because removal of apical buds elicited the response although removal of the same number of basal buds did not. Induced amelioration is a different response to induced resistance. The two responses are triggered by different cues and may occur in the same plant. By altering hormonal balance of shoots it is potentially possible for herbivores to induce amelioration of food quality. The ways in which herbivory is simulated may explain variability of results obtained when herbivory-induced responses in plants have been studied.

Seed bank versus seed rain in the regeneration of a tropical pioneer tree.

Abstract: We used the tropical pioneer tree, Cecropia obtusifolia to evaluate the relative importance of different sources of seeds in the regeneration of species that depend on ephemeral sites. We studied seed production in a population established in a 5 ha plot, and dispersal, dormancy and seed predation in two recent treefall gaps ( 35 years since disturbed) for a one year period at Los Tuxtlas (Mexico). Flowers and fruits were counted at monthly intervals. Annual fecundity per tree ranged from 1.4×104 to 1.4×107 seeds. Seeds were continuously available on the trees and on the ground. Average annual seed rain per m2 (as measured by 0.5×0.5 m seed traps) varied from 184 to 1925 among the six sites. Distance to nearest seed source and patch type explained more than 60% of the seed rain variation among sites. Soil seed density, estimated by counting seeds from ten samples (78.5 cm2×10 cm deep) collected from each site in October and January, ranged among the six sites from 269 to 4485 seeds per m2 in January and from 204 to 5073 in October. Soil seed viabilities were much lower (17.1% in October and 5.1% in January) than those of rain seeds (48.26%). Annual survivorships of 2.2% were estimated for seeds artificially sown on the soil surface of a gap and a mature patch, and 3.75% in a building patch. In two other experiments seed removal rates ranged from 27% to 98% in 4 days. Removal rates were significantly higher in gap and mature patches than in building patches. Ants (Paratrechina vividula) and grasshopper nymphs (Hygronemobius. sp.) were the main predators. We draw three main conclusions from our data: (1) Pathogens and predators determine low survivorship of C. obtusifolia's seeds in the soil and a rapid turnover rate (1.07 to 1.02 years) of its seed bank; (2) a continuous and copious seed production and an abundant and extensive seed rain replenish the soil seed pool in patches with different disturbance ages at least up to 86 m from nearest source; (3) more than 90% of the seeds contributing to C. obtusifolia seedling recruitment in gaps are less than one year-old. We discuss our results in the context of previous similar studies for tropical forests.

Plant structural complexity and host-finding by a parasitoid.

Abstract: There are three major components to plant structure relevant to searching parasitoids: 1) plant size or surface area, 2) the variation among plant parts (structural heterogeneity), such as seed heads, flowers and nectaries, and heterogeneous surfaces (e.g. glabrous, hirsute), and 3) the connectivity of parts or plant form (structural complexity). We examined the effect of structural complexity, while controlling for size and structural heterogeneity, on searching behaviors of Trichogramma nubilale in controlled environments. Females were presented with a structurally simple surface and a structurally complex one. Parasitism rates were 2.9 times higher on simple surfaces than on complex ones. Unexpectedly, when no hosts were present, searching time on simple surfaces was 1.2 times higher than on complex surfaces. This implies that structural complexity per se can affect the giving-up-time of a searching parasitoid. Searching efficiency, however, was the dominant process, and females found hosts on simple surfaces 2.4 times faster than on complex surfaces. Structural complexity can have a dramatic effect on the success of parasitoid search.

Carbon balance in tussock tundra under ambient and elevated atmospheric CO2.

Abstract: Whole ecosystem CO2 flux under ambient (340 μl/l) and elevated (680 μl/l) CO2 was measured in situ in Eriophorum tussock tundra on the North Slope of Alaska. Elevated CO2 resulted in greater carbon acquisition than control treatments and there was a net loss of CO2 under ambient conditions at this upland tundra site. These measurements indicate a current loss of carbon from upland tundra, possibly the result of recent climatic changes. Elevated CO2 for the duration of one growing season appeared to delay the onset of dormancy and resulted in approximately 10 additional days of positive ecosystem flux. Homeostatic adjustment of ecosystem CO2 flux (sum of species' response) was apparent by the third week of exposure to elevated CO2. Ecosystem dark respiration rates were not significantly higher at elevated CO2 levels. Rapid homeostatic adjustment to elevated CO2 may limit carbon uptake in upland tundra. Abiotic factors were evaluated as predictors of ecosystem CO2 flux. For chambers exposed to ambient and elevated CO2 levels for the duration of the growing season, seasonality (Julian day) was the best predictor of ecosystem CO2 flux at both ambient and elevated CO2 levels. Light (PAR), soil temperature, and air temperature were also predictive of seasonal ecosystem flux, but only at elevated CO2 levels. At any combination of physical conditions, flux of the elevated CO2 treatment was greater than that at ambient. In short-term manipulations of CO2, tundra exposed to elevated CO2 had threefold greater carbon gain, and had one half the ecosystem level, light compensation point when compared to ambient CO2 treatments. Elevated CO2-acclimated tundra had twofold greater carbon gain compared to ambient treatments, but there was no difference in ecosystem level, light compensation point between elevated and ambient CO2 treatments. The predicted future increases in cloudiness could substantially decrease the effect of elevated atmospheric CO2 on net ecosystem carbon budget. These analyses suggest little if any long-term stimulation of ecosystem carbon acquisition by increases in atmospheric CO2.

Large herbivores that strive mightily but eat and drink as friends

Abstract: Grazing in patches of Cynodon dactylon and of Sporobolus spicatus by four large herbivores, and the interaction between these sedentary herbivores was studied in Lake Manyara National Park, northern Tanzania. The herbivores were the African buffalo, Syncerus caffer; the African elephan, Loxodonta africana; the Burchell's zebra, Equus burchelli; and the wildebeest, Connochaetus taurinus. Four different hypotheses of the interactions between the herbivores were tested, viz., increased predator detection/protection through association of species, facilitation of the food intake through the influence of other species, use by other species of the food manipulation strategy of buffalo, and interspecific competition for food. On the level of a single day, zebra and wildebeest were symbiotic, which could have been caused by an increased chance of predator detection. A similar association between buffalo and wildebeest or zebra was also detected on C. dactylon grasslands. There was no indication of facilitation between any of the herbivores. Buffalo had a despotic relationship with elephant, that is the elephant's consumption was lowered when buffalo had visited a patch prior to their arrival. When elephant and buffalo arrived at the same time there appeared to be scramble competition between them. Habitat overlap was calculated for four pairs of species. In conjunction with the analyses of the patch visits, it was concluded that a small overlap was associated with interspecific competition and a large habitat overlap was associated with symbiosis.

Climatic change and its ecological implications at a subantarctic island.

Abstract: Marion Island (47°S, 38°E) has one of the most oceanic climates on earth, with consistently low air temperatures, high precipitation, constantly high humidity, and low incident radiation. Since 1968 mean surface air temperature has increased significantly, by 0.025° C year-1. This was strongly associated with corresponding changes in sea surface temperature but only weakly, or not at all, with variations in radiation and precipitation. We suggest that changing sealevel (atmospheric and oceanic) circulation patterns in the region underlie all of these changes. Sub-Antarctic terrestrial ecosystems are characterized by being species-poor and having a simple trophic structure. Marion Island is no exception and a scenario is presented of the implications of climatic change for the structure and functioning of its ecosystem. Primary production on the island is high and consequently the vegetation has a large annual requirement for nutrients. There are no macroherbivores and even the insects play only a small role as herbivores, so most of the energy and nutrients incorporated in primary production go through a detritus, rather than grazing, cycle. Ameliorating temperatures and increasing CO2 levels are expected to increase productivity and nutrient demand even further. However, most of the plant communities occur on soils which have especially low available levels of nutrients and nutrient mineralization from organic reserves is the main bottleneck in nutrient cycling and primary production. Increasing temperatures will not significantly enhance microbially-mediated mineralization rates since soil microbiological processes on the island are strongly limited by waterlogging, rather than by temperature. The island supports large numbers of soil macro-arthropods, which are responsible for most of the nutrient release from peat and litter. The activities of these animals are strongly temperature dependent and increasing temperature will result in enhanced nutrient availability, allowing the potential for increased primary production due to elevated temperature and CO2 levels to be realized. However, housemice occur on the island and have an important influence on the ecosystem, mainly by feeding on soil invertebrates. The mouse population is strongly temperature-limited and appears to be increasing, possibly as a result of ameliorating temperatures. We suggest that an increasing mouse population, through enhanced predation pressure on soil invertebrates, will decrease overall rates of nutrient cycling and cause imbalances between primary production and decomposition. This, along with more direct effects of mice (e.g. granivory) has important implications for vegetation succession and ecosystem structure and functioning on the island. Some of these are already apparent from comparisons with nearby Prince Edward Island where mice do not occur. Other implications of climatic change for the island are presented which emphasize the very marked influences that invasive organisms have on ecosystem structure and functioning. We suggest that changing sealevel circulation patterns, by allowing opportunities for colonization by new biota, may have an even more important influence on terrestrial sub-Antarctic ecosystems than is suggested merely on the basis of associated changes in temperature or precipitation.

Underestimation of mutual interference of predators.

Abstract: The usual method of estimating the mutual interference constant, m, assumes a linear (type I) functional response of predators. In the cases where the response is not linear, the application of the method introduces a bias in the estimation of the searching efficiencies. It is shown that, as a consequence, the value of m is underestimated. A new method is proposed, which allows for a type II functional response due to a handling time. A comparative analysis of 15 data sets from the literature shows that the proposed method gives values of m that are consistently higher than those estimated by the traditional method. The new method calculates the parameters with nonlinear regression and provides standard errors for the estimates. Therefore, the reliability of the searching efficiencies, the handling time and the constant m can be quantified. Very few of the interference constants are significantly different from m=1. This special value implies that the functional response is a function of the ratio of prey and predator densities. These empirical findings support the suggestion of Arditi and Ginzburg (1989) that the functional response might often be ratio-dependent, especially in complex and heterogeneous situations.

Marine epibiosis. II. Reduced fouling on Polysyncraton lacazei (Didemnidae, Tunicata) and proposal of an antifouling potential index

Abstract: Polysyncraton lacazei is a colonial tunicate (family didemnidae) living in the NW-mediterranean rocky sublitoral. A thorough scanning of numerous colonies revealed that in spite of an apparently heavy local fouling pressure only one fouling species - a kamptozoan - is encountered with some regularity on Polysyncraton. We try to define the epibiotic situation of sessile marine organisms as composed of four epibiotic parameters: longevity or exposure time (A), epibiont load (E), colonizer pool (CP) and fouling-period (FP). Subsequently, these factors are combined to propose an "Antifouling Potential" index: AFP=(1-E/CP)×A/(FP+A). This index is intended to permit evaluating the relative antifouling defense potency to be expected in a given organism in a given epibiotic situation and to compare different cases of epibiosis and fouling.

Genetic variation in and covariation between leaf gas exchange, morphology, and development in Polygonum arenastrum, an annual plant

Abstract: We present evidence of genetic variation in and covariation between leaf-level gas exchange properties and leaf size among family lines of Polygonum arenastrum. This self-fertilizing annual had previously been shown to vary genetically in developmental phenology and in morphology (size of leaves, internodes, flowers and seeds) (Geber 1990). Significant family differences were found in photosynthetic carbon assimilation rate (A), lcaf conductance to water vapor (g), instantaneous water-use efficiency (WUE), and leaf carbon isotope discrimination (Δ). A strong positive genetic correlation between A and g suggested that there was stomatal limitation on A. In addition, higher g led to relatively greater increases in transpiration, E, than in assimilation, A, so that families with high rates of gas exchange had lower instantaneous WUE and/or higher carbon isotope discrimination values. Leaf size and gas exchange were genetically correlated. In earlier studies leaf size was found to be genetically correlated with developmental phenology (Geber 1990). The pattern that emerges is one in which small-leaved families (which also have small internodes, flowers, and seeds) tend to have high gas exchange rates, low WUE, rapid development to flowering and high early fecundity, but reduced life span and maximum (vegetative and reproductive) yield compared to large-leaved families. We suggest that this pattern may have arisen from selection for contrasting suites of characters adapted to environments differing in season length.

Plant competition for light analyzed with a multispecies canopy model : I. Model development and influence of enhanced UV-B conditions on photosynthesis in mixed wheat and wild oat canopies.

Abstract: Competition for light among species in a mixed canopy can be assessed quantitatively by a simulation model which evaluates the importance of different morphological and photosynthetic characteristics of each species. A model was developed that simulates how the foliage of all species attenuate radiation in the canopy and how much radiation is received by foliage of each species. The model can account for different kinds of foliage (leaf blades, stems, etc.) for each species. The photosynthesis and transpiration for sunlit and shaded foliage of each species is also computed for different layers in the canopy. The model is an extension of previously described single-species canopy photosynthesis simulation models. Model predictions of the fraction of foliage sunlit and interception of light by sunlit and shaded foliage for monoculture and mixed canopies of wheat (Triticum aestivum) and wild oat (Avena fatua) in the field compared very well with measured values. The model was used to calculate light interception and canopy photosynthesis for both species of wheat/wild oat mixtures grown under normal solar and enhanced ultraviolet-B (290–320 nm) radiation (UV-B) in a glasshouse experiment with no root competition. In these experiments, measurements showed that the mixtures receiving enhanced UV-B radiation had a greater proportion of the total foliage area composed of wheat compared to mixtures in the control treatments. The difference in species foliage area and its position in the canopy resulted in a calculated increase in the portion of total canopy radiation interception and photosynthesis by wheat. This, in turn, is consistent with greater canopy biomass of wheat reported in canopies irradiated with supplemental UV-B.

A survey of vessel dimensions in stems of tropical lianas and other growth forms

Abstract: Vessel dimensions (total diameter and length) were determined in tropical and subtropical plants of different growth forms with an emphasis upon lianas (woody vines). The paint infusion and compressed air methods were used on 38 species from 26 genera and 16 families in the most extensive survey of vessel length made to date. Within most stems there was a skewed frequency distribution of vessel lengths and diameter, with many short and narrow vessels and few long and wide ones. The longest vessel found (7.73 m) was in a stem of the liana (woody vine) Pithecoctenium crucigerum. Mean vessel length for 33 species of lianas was 0.38 m, average maximum length was 1.45 m. There was a statistically significant inter-species correlation between maximum vessel length and maximum vessel diameter. Among liana stems and among tree+shrub stems there were statistically significant correlations between stem xylem diameter and vessel dimensions. Lianas with different adaptations for climbing (tendril climbers, twiners, scramblers) were similar in their vessel dimensions except that scramblers tended to have shorter (but not narrower) vessels. Within one genus, Bauhinia, tendril climbing species had greater maximum vessel lengths and diameters than tree and shrub species. The few long and wide vessels of lianas are thought to hydraulically compensate for their narrow stem diameters. The many narrow and short vessels, which are present in the same liana stems, may provide a high resistance auxiliary transport system.

Preference of cabbage white butterflies and honey bees for nectar that contains amino acids

Abstract: Amino acids occur in most floral nectars but their role in pollinator attraction is relatively unstudied. Nectars of butterfly-pollinated flower tend to have higher concentrations of amino acids than do flowers pollinated by bees and many other animals, suggesting that amino acids are important attractants of butterflies to flowers. In order to determine whether amino acids are important in attracting butterflies and bees, we tested the preference of cabbage white butterflies (Pieris rapae) and honey bees (Apis mellifera) by allowing them to feed from artificial flowers containing sugar-only or sugar-amino acid mimics ofLantana camara nectar. Honey bees and female cabbage white butterflies consumed more sugar-amino acid nectar than sugar-only nectar. In addition, female cabbage white butterflies visited artificial flowers containing sugar-amino acid nectars more frequently than flowers containing sugar-only nectars; honey bees spent more time consuming the sugar-amino acid nectar. Male cabbage white butterflies did not discriminate between the two nectars. These results support the hypothesis that the amino acids of nectar contribute to pollinator attraction and/or feeding.